1. Field of the Invention
The present invention relates to a plasma display apparatus, and relates in particular to a plasma display apparatus which has an improved data converter and which can display a higher quality image.
2. Related Arts
A plasma display apparatus is so designed that a predetermined number of electrodes are formed on paired, facing substrates separated by a discharge space, that plasma discharges are generated between opposed electrodes, that phosphors formed on the substrates are excited by ultraviolet rays produced by the plasma discharges, and that a predetermined image is displayed. Furthermore, the number of discharges in each frame is controlled, so that an image having a desired luminance can be displayed.
FIG. 14 is a diagram for explaining a sub-frame system for a conventional three-electrodes surface discharge plasma display panel. With a three-electrode surface discharge AC plasma display, one frame is divided into a plurality of sub-frames SF, and the luminance level is represented by the ratio for the numbers of the sustain discharge pulses in the sub-frames. Provided for each sub-frame SF are: a reset period RST, for resetting a whole panel; an address period ADD, during which a cell to be displayed is discharged in accordance with desired image data and wall charges are accumulated; and a sustain period SUS, during which, for a cell wherein wall charges are accumulated when a discharge occurs during the address period ADD, sustain discharging is repeated the number of times which corresponds to the ratio for the luminance level of the sub-frame.
In the example in FIG. 14, one frame is formed of eight sub-frames SF1 to SF8, and the ratio for the sustain discharges of the sub-frames SF1 to SF8 is set as 1:2:4:8:16:32:64:128. Accordingly, the same ratio is applied to the luminance represented by the sub-frames. Thus, the desired luminance for one frame can be displayed in accordance with a predetermined combination of sub-frames.
Video data (or image data) input to a plasma display apparatus are normally digital data designating gray scale of RGB colors in individual frames. Therefore, the plasma display apparatus includes a data converter for converting the gray scale color data of each frame into display data which are constituted by combinations of sub-frames. Generally, the data converter can be implemented by a conversion table.
FIG. 15 is an example conversion table for a conventional data converter, and FIGS. 16A and 16B are graphs showing the characteristic of the conversion table in FIG. 15. In the examples of FIGS. 15 and 16, eight-bit input video data having 256 gray scales are converted into output display data which specify the ON/OFF states of the eight sub-frames SF1 to SF8, so that a luminance level of 0 to 255 is displayed within one frame. For example, when the gray scale level of the input video data is 7, the sub-frames SF1(1), SF2(2) and SF3(4) are turned on, and when the gray scale level of the input video data is 255, all the sub-frames SF1(1) to SF8(128) are turned on.
As is shown in FIG. 16A, the characteristic of the conventional conversion table is a linear function, according to which the luminance level of the output display data is incremented by one when the gray scale of the input video data is incremented by one, and corresponds to a binary counter value. In accordance with the characteristic, in the conversion table in FIG. 15 the luminance levels 0 to 255 of the output display data respectively correspond to the gray scale levels of 0 to 255 for the input video data. In FIG. 16B, a partially detailed characteristic curve is shown which corresponds to the conversion table in FIG. 15, and in all the areas wherein the gray scale levels of the input video data are from close to 0 up to 255, the luminance level of the output display data is incremented by one at the same time the gray scale of the input video data is incremented by one.
The conversion table or the conversion characteristic of the conventional data converter depicts a linear function whereby the luminance level is uniformly increased or reduced in all gray scale areas wherein the input video data is used. However, according to this conversion characteristic, in a low luminance area the number of luminance levels to which the sight of a person is most sensitive is insufficient, and the luminance level for a dark image is unsatisfactory. That is, the luminance resolution for a dark image is not satisfactory. On the other hand, since in a high luminance area the sight of a person is not as sensitive as it is in a low luminance area, in a high luminance area there are more levels than is necessary.
Furthermore, externally input video data tend to be modulated by gamma compensation from an original video signal. The gamma compensation for such a video signal is performed in accordance with the gamma characteristic of a display device, such as CRT which has more luminance levels in a low input area. However, since the plasma display apparatus having the conversion characteristic shown in FIGS. 15 and 16 does not have such a gamma characteristic, for the input video data with gamma compensation, the gray scale levels can not be represented appropriately in a low luminance area, so luminance resolution in the low luminance area is lost.
In addition, since the conventional data converter employs the same conversion table or conversion characteristic shown in FIGS. 15 and 16 for the three primary colors, RGB, the color balance (tint) can not be changed. Generally, a display image having a low color temperature, such as reddish white, is preferred in the Europe and U.S.A., while an image having a high color temperature, such as bluish white, is preferred in Japan. However, the conventional plasma display apparatus can not provide with the difference between the color temperatures. Further, because of the characteristics of the phosphors, a different luminance may be produced for each color for a predetermined same sustain discharge count.